In the case of a rigid-frame vehicle, the techniques used for vehicle steering are straightforward and well-known. No doubt, the most common arrangement is to steer that pair of wheels which, for the usual direction of travel, is forwardmost. The axles of the trailing wheels are fixed with respect to the vehicle body.
Less common but by no means unheard of is an arrangement in which a rigid-frame vehicle has four wheels, two on each of two steerable axles. The wheel pairs are steerable, either in opposite directions or in the same direction. The latter is known as "crab" steering.
Other types of vehicles are articulated; that is, such vehicles have a pair of rigid frames pinned together for relative pivoting movement with respect to one another. Certain types of articulated construction vehicles are configured in this way and use a pair of hydraulic cylinders to apply force to the frames and change the angle of such frames relative to one another.
Another example of an articulated vehicle, commonly known as a "semi," is an over-the-road haulage trailer coupled to a towing tractor. Trailer-tractor "jackknifing" is relatively common and a good deal of creative effort had been directed to arrangements to limit the angle that such trailer and tractor can assume with respect to one another. Examples are disclosed in U.S. Pat. No. 3,701,547 (Goold) and U.S. Pat. No. 4,366,966 (Ratsko et al.).
U.S. Pat. No. 5,244,226 (Bergh) discloses a system for steering the wheels of a towed, two-wheel trailer in response to variations in the angle of articulation between the trailer and the steerable towing vehicle. The system compares the angle of articulation and the angle of the trailer wheels, both as a percent of some maximum (although not necessarily equal) angle. The angle of the trailer wheels is the dependent variable and is changed as a function of the angle of articulation.
None of the foregoing examples contemplate (or seem to contemplate) mobile equipment having several trailer-like vehicles pivotally pinned together to form what might be described as a "train." Mobile equipment of this type is common in underground mines for transporting, i.e., hauling or conveying, the mined product.
Such mining mobile equipment may be characterized in three ways. One is that the number of vehicles pivotally-pinned together may be relatively high, e.g., ten, a dozen or more. Another is that such equipment is routinely moved through relatively narrow, curving tunnels. And another characterization is that the equipment tends to undulate or "concertina" as it is moved along a tunnel. That is, each vehicle does not necessarily faithfully follow the tunnel roadway but, rather, may move from side to side, i.e., transversely with respect to the desired line of travel, as the train of vehicles moves. This makes the equipment more difficult to steer and, in fact, imposes undesirable loads on the towing gear.
And there is another factor which makes vehicle undulation undesirable. In an underground mine, it is common to leave in place a number of vertical columns (sometimes known as pillars) or ribs of the material being mined, e.g., coal. Left-in-place pillars and ribs help support the roof and prevent mine collapse.
An undulating material transport system risks striking one of the columns or ribs and, perhaps, weakening it. As a result, room support is impaired, at least to some degree.
A steering system and method useful on material transport equipment and which address problems and shortcomings of the prior art would be an important technological advance.